I’d want either an RPG or a phaser.
Good gad.
I want some.
Huh. This would be near my last choice for a weapon, actually. (Better than a spear, but not by a whole lot.) Relatively short-range, and while flame is certainly going to be painful, is it going to cook through that much flesh fast enough to incapacitate or kill? Yes, I’m sure it will be painful - but I suspect t-rex (or any animal that feeds on large prey, for that matter) is going to be used to the idea that Dinner Sometimes Hurts. That might not make him stop trying to eat you.
Actually, predators, especially nonsocial predators, back off potential injury far more than nonpredators, because any injury that makes it unable to hunt would make it starve. Better to be safe than sorry and wait until you can get an easy meal. Now the question is whether the T Rex would recognize the flame as a threat or not. My thoughts are that it would.
ETA that is if you turn on the flamer well before the T Rex attacks you. If you only use it as a weapon to hurt it rather than as a warning, the dinosaur might maul you to death in the heat of the moment (see what I did there?) before it backed off due to injury.
Sir, you have fought my ignorance - thank you! If I might trouble you a bit further, would you be so kind as to provide a cite?
Maybe we need to clarify the situation. Are we hunting for T-Rex safari-style? Are we a hardy group of time travelling explorers who have stumbled upon one at its kill and caused it to attack us? Are we attempting to bag a specimen for a museum? Depending on what we are about, the choice of weapons is going to vary dramatically. A big-ass, high powered dart gun that dumps a massive overload of some hyper-deadly neurotoxin would surely kill one, maybe even without it realizing it’s been shot; but is that in the spirit of the OP?
The most important thing is not the caliber of the gun; that’s secondary. The most important thing is becoming an expert on the animal’s anatomy, and knowing exactly where the brain is. That’s how you bring 'em down.
I’m more interested in preventing Mr Rex from eating me than killing it, so I see it as part deterrance and part weapon. There are of course a lot of unknowns concerning T. Rex behaviour, but I’m assuming that it won’t run through a wall of flame. Military flamethrowers have a range of over 200 feet, much further than is commonly assumed. If it persists in attacking, burns would further discourage it. As Ludovic says, predators are generally risk-averse. In any case, it’s not known if T. Rex was primarily a scavenger or a predator, and how aggressive it might have been.
In terms of killing a T. Rex as the OP asked, a flamethrower would be pretty effective, as large burns would become infected. A high-power rifle would probably work better if you have time to aim and take several shots, but a flamethrower would make a good defensive weapon.
I’m unlikely to get a chance to test any of this, as I never get invites to newly opened theme parks. 
If I’m ever in a position to be killing a T. rex, I’ll want something that kills a lot faster than infection. Or at least, incapacitates: Hamstringing might be good enough.
You see it a lot in documentaries. In one I was watching a while back, a cheetah had snagged something. While it was in the process of chowing down, a leopard wandered by. A cheetah is decidedly outgunned in a match with a leopard, but after a few dirty looks back and forth, the leopard went on it’s way. The narrator voice-over pretty much saying that to the leopard, it just wasn’t worth it.
But you also see that sort of thing a lot in documentaries. Often you’ll see a lot of posturing and displaying, and someone backs down before any contact actually occurs, inter-species and intra-species.
Maybe you could feed him a bunch of ice-cream really fast, give him an ice-cream headache
Something that isn’t often taken into account in the threads on this question is that a t-rex is so huge with a clear line of sight it would be clearly visible at least 2 miles away and probably more. It is like trying to shoot a 2 story building. There is a lot of debate about how fast a t-rex could run (or if they could run at all) but most estimates range between about 11Mph at the slowest and 30Mph at the fastest. So let’s say they could travel at 20Mph. If you could line up a clear shot from 2 to 3 miles away, especially from a high vantage point that is an uphill run for him, you would have 5 or 10 minutes after the first shot before he could reach you. At that range there is even a chance he would have no idea where the attack was coming from, and you might have unlimited time/shots. I think I would prefer a very long range round and lots of ammo in the hope of disabling him before he got close enough for one of these big game rounds to even be effective.
The one thing you are absolutely positively not going to be able to do is hit a Tyrannosaur in the brain. Its head is four feet long. Its brain is bigger than a walnut, but not by much, and buried under layers of muscle, skin, bone, air spaces, teeth, and sinus. Trying to hit it in the brain is like trying to fight a mugger by shooting the knife out of his hand.
Now, maybe shooting it in the face is a good way to scare it away. There’s probably a lot of sensitive tissue there. Or it might get mad. Tough call. But you’re not going to hit the brain.
How is that note related??!!? 
:dubious: 
The really important thing is: don’t step on any butterflies.
I’m not sure, but I’d say they would be short work for a team of raptors on hoverboards.
Thank God somebody reads the column.
Not only was T-Rex huge but it’s brain would be a small target buried in the back of a big bony head, it might have had osteoderms (bony plating) in it’s skin and gastralia- bones armoring the front of it’s torso. Definitely maximum penetration is called for here.
This round and the guns that shoot it sounds like a contender: .460 Weatherby Magnum
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Who has the Alamosaurus?